Primordial germ cells are the embryonic precursors of the gametes. Errorscan affect PGC behavior in culture. To determine what role this cytokine family plays in PGC development, animals carrying mutations in the receptor (gp130) or a downstream component of this signaling pathway (STAT3) will be bred with a line of mice that express green fluorescent protein in the germ cell lineage. The positions and numbers of germ cells will be determined in tissue sections taken from these mutant animals. Also, mutant PGCs will be isolated by FACs and assayed for their ability to survive, divide and migrate in culture. Finally, conditional gene targeting will be used to control the timing and tissue specificity of gp130/STAT3 activity. This will help to define when gp130 signaling is required and whether its effects are cell autonomous during PGC development. This analysis will provide novel insights into the factors that influence vertebrate fertility.